If \[ y = 500 e^{7x} + 600 e^{-7x}, \quad \text{then show that} \quad \frac{d^2 y}{dx^2} = 49y. \]
Solve the differential equation \[ \frac{dy}{dx} + y \cot x = 2x + x^2 \cot x \quad \text{where} \quad x \neq 0. \]
Solve the differential equation \[ (x - y) \frac{dy}{dx} = x + 2y. \]
Minimize Z = 5x + 3y \text{ subject to the constraints} \[ 4x + y \geq 80, \quad x + 5y \geq 115, \quad 3x + 2y \leq 150, \quad x \geq 0, \quad y \geq 0. \]